In recent years, in the medical and graphic arts fields, a decrease in the processing effluent has been increasingly demanded from the viewpoint of environmental protection as well as space saving.
As a result, techniques have been sought which relate to photothermographic materials which can be effectively exposed, employing laser imagers and laser image setters, and can form clear black-and-white images exhibiting high resolution.
Such techniques are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,487,075, both by D. Morgan and B. Shely, or D. H. Klosterboer et al., “Dry Silver Photographic Materials”, (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991). Also known are silver salt photothermographic dry imaging materials (hereinafter occasionally referred to simply as photothermographic materials) which comprise a support having thereon organic silver salts, photosensitive silver halide and reducing agents. Since any solution-based processing chemicals are not employed for the aforesaid silver salt photothermographic dry imaging materials, they exhibit advantages in that it is possible to provide a simpler environmentally friendly system to customers.
These silver salt photothermographic dry imaging materials are characterized in that photosensitive silver halide grains, which are incorporated in a photosensitive layer, are utilized as a photo-sensor and images are formed in such a manner that silver halide grains are thermally developed, commonly at 80 to 140° C., utilizing the incorporated reducing agents while using organic silver salts as a supply source of silver ions, and fixing need not be carried out.
However, the aforesaid silver salt photothermographic dry imaging materials tend to result in fogging during storage prior to thermal development, due to incorporation of organic silver salts, photosensitive silver halide grains and reducing agents. Further, after exposure, thermal development is commonly carried out at 80 to 250° C. followed by no fixing. Therefore, since all or some of the silver halide, organic silver salts, and reducing agents remain after thermal development, problems occur in which, during extended storage, image quality such as silver image tone tends to vary due to formation of metallic silver by heat as well as light.
Techniques which overcome these problems are disclosed in Patent Documents Nos. 1 and 2 employing vinyl sulfone compounds or photo oxidation compounds. These techniques disclosed therein exhibit some effects, but are not fully sufficient to meet the market's requirements.
In addition, for the purpose of enhancing covering power(CP), when the number of photosensitive silver halide grains is increased while decreasing the diameter of the aforesaid grains, it has been found that problems occur in which variation and degradation of image quality such as tone of silver images are further accelerated due to effects of light incident to the aforesaid photosensitive slier halide grains during storage of the aforesaid photosensitive silver halide grains after development as well as while viewing them.
A technology employing a leuco dye capable of producing color is disclosed. This technology enables to adjust a hue of silver to a preferred color. The hue of silver is caused by a morphology of silver. Examples of such technology are disclosed in Japanese Patent Publication Open to Public Inspection (hereafter it is referred to as JP-A) Nos. 50-36110, 59-206831, 5-204087, 11-231460, 20002-169249 and 2002-236334. However, this technology is not fully effective to prevent change of color of silver after long-term storage.
It is disclosed another technology to prevent change and deterioration of silver caused by irradiation of light. That technology employs a halogenated compound capable of oxidizing a silver image by irradiation of light. Examples of compounds are shown in Patent Documents Nos. 3 and 4. However, these compounds generally tend to exhibit an oxidizing property by an effect of heat. As a result, they have an effect of preventing fog formation but at the same time they may prevent formation of a silver image resulting in a loss of photographic speed, a loss of Dmax and a loss of a silver covering power.
On the other hand, these silver salt photothermographic dry imaging materials always incorporate developing agents, organic carboxylic acid silver salts as a silver supplying source, and light-sensitive silver halide. As a result, not only storage stability prior to exposure but also that of after thermal development results in major problems.
Disclosed as techniques to enhance stability of these silver salt photothermographic dry imaging materials is one in which with regard to cores and shells of organic carboxylic acid silver salt particles, particles are subjected to formation of core/shell, and by changing the silver salt composition of the surface from that of the interior, developability at relatively low temperature is improved to result in high Dmax (refer, for example to Patent Document 5). However, it was discovered that when the silver salt composition of the surface was only changed from that of the interior, stability was degraded, whereby storage stability was also occasionally deteriorated.
On the other hand, disclosed as a technique to enhance stability of silver salt photothermographic dry imaging materials is one in which employed as light-sensitive silver halide grains are those which are surface-sensitive prior to thermal development and become an internal image forming type (refer, for example, to Patent Document 6). This is an epoch-making technique in which after thermal development, light-sensitive silver halide grains are subjected to be of an internal image forming type to result in rapid decrease in surface photographic speed, whereby even though silver salt photothermographic drying imaging materials are exposed to natural light, no fog is formed and storage stability of images is improved. However, problems occur in which depending on storage conditions prior to exposure, photographic speed markedly decreases especially during storage at relatively high humidity.
On the other hand, demanded as so-called “eternal object” is further improvement of image quality. Specifically, in the medical image field, demanded is development of techniques to achieve higher quality images to enable more accurate diagnosis.
It is demanded to develop a new and high technology to achieve a high image quality in order to solve the above-described problems in the imaging materials of the present technical field.
Patent Document No. 1: JP-A No. 6-208192
Patent Document No. 2: JP-A No. 8-267934
Patent Document No. 3: JP-A No. 7-2781
Patent Document No. 4: JP-A No. 6-208193
Patent Document No. 5: JP-A No. 2002-23303
Patent Document No. 6: JP-A No. 2003-270755